Scottmel, I think I know what you mean, I myself have had knee surgeries as results of torn cartilage and was in a accident with a fractured hip. All this seems to affect my lower back, especially when sitting for a long time. I find that my posture and keeping a good posture when sitting helps more than anything.
I suggest is first understand what your best seat depth should be; measure from the inside of your knee to your back while your sitting in a good posture. This should give you a basic starting point to work from. Best to have the seats and back pillow in foam encased inside a envelope or wrapped that has no more than 3/4" thickness of polyester fiber or memory foam around the foam. The seat foam should be a HR foam 2.0 or higher density and a ILD of 28 - 32. This is a standard Firm seat that will remain the same firmness for many years. The back cushions should have no less than 1.8 density and 18- 22 ILD. A tight back sofa is fine too if you can find the one with the right depth to your size. A pillow back will allow for a easier adjustment for the manufacturer.
I think your best option is to find a custom built sofa to fit your requirements. One that offers a number of choices for seating comfort ie: a firmer seat.
Of course I built my own sofa specifically for my hips and knees, well because I am the owner of a upholstery plant.
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seating heiht for bad knees, hips
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